1. Field of the Invention
The present invention relates to a technology for driving a display panel constituted by a group of memory cells having a memory function as display elements. More particularly, the present invention relates to a method for driving a plasma display panel, which is directed to reducing background light emission of an alternating current (AC) type plasma display panel. (Generally, a plasma display apparatus, inclusive of the plasma display panel is referred to as a xe2x80x9cPDPxe2x80x9d.)
The AC type plasma display panel, of this kind sustains discharges and carries out light emission display by alternately applying voltage waveforms of a plurality of pulses to two electrodes for sustaining this discharge (i.e., sustain electrodes). A discharge (lighting) operation for every discharge period finishes within a few micro-seconds (is) after the application of pulses. Ions defined as positive charges that are generated by this discharge are accumulated over an insulating layer on the electrode to which a negative voltage is applied. Similarly, electrons as negative charges are accumulated over an insulating layer on the electrode to which a positive voltage is applied.
Therefore, when wall charges are generated by causing the discharges by the pulses, (write pulses) each having a relatively high voltage, (write voltage) and then the pulses (sustain discharge pulses, that is, xe2x80x9csustain pulsesxe2x80x9d), each having a voltage lower than that of each of the write pulses (sustain discharge voltage) and an opposite polarity to each of the write pulses, are applied to the electrodes, electric charges generated by the sustain pulses are superimposed on the wall charges previously accumulated by the write pulses so as to enhance the accumulated wall charges.* As a result, the potential of the wall charges with respect to a discharge space becomes large and, eventually the above voltage exceeds a discharge threshold voltage at which the discharge starts. In other words, given cells that once executed the write discharge and have formed the wall charges have characteristics such that of these cells sustain the discharge when the sustain discharge pulses are alternately applied thereto in the opposite polarities. A phenomenon having the above characteristics is referred to as a xe2x80x9cmemory effectxe2x80x9dor xe2x80x9cmemory drivexe2x80x9d. The AC type plasma display panel carries out display by utilizing this memory effect.
2. Description of the Related Art
The AC type plasma display panels can be Hi . *classified into a two-electrode type which executes selective discharge. (i.e., selective address discharge) and sustain discharge by two electrodes, and a three electrode type which executes the addressing discharge by Utilizing a third-electrode. In color plasma display panels for effecting multi-gradation display., a phosphor inside the cell is excited by ultra-violet rays generated by the discharge between different kinds of electrodes, but this phosphor involves the problem that it is extremely fragile against the impact of the ions defined as the positive charges that are generated simultaneously by the discharge (that is, the phosphor is sensitive to the impact of the ions). Since the former two-electrode type plasma display panel described above employs the construction in which the ions are allowed to-collide directly with the phosphor, the life of the phosphor is likely to become shorter. To avoid this problem, the latter three-electrode type plasma display panel utilizing a surface discharge (that is, a surface discharge type plasma display panel which is carried out between, different electrodes that are located in the same plane), has been used generally in the color plasma display panels.
Here, in order to enable the problems of the driving method of the plasma display panel, according to the prior art systems, to be more easily understood, the construction of a conventional plasma display panel and its driving method will be explained with reference to FIGS. 1 to 3 of the later-appearing xe2x80x9cBrief Description of the Drawingsxe2x80x9d.
An AC type color plasma display panel which is a three-electrode and a surface discharge type, such as the one shown in a schematic plan view of FIG. 1, has been known in the past., In FIG. 2, a schematic sectional view of cells shown in FIG. 1 in a horizontal direction is illustrated.
A panel 1 comprises two glass substrates (that is, a front glass substrate 8 and a back glass substrate 9). The front glass substrate 8 defined as the first glass substrate includes first and second electrodes (X electrodes 2, Y electrodes 3-1 to 3-N (where N is an arbitrary positive integer of 2 or more than 2)), which are both defined as parallel sustain electrodes. Each of these electrodes comprises a transparent electrode 14 and a bus electrode 13.. The transparent electrode 14 is made of an ITO (a transparent conductive film consisting of indium oxide as the main component), etc., because it has a role of transmitting the reflected rays of light from the phosphor 12. The bus electrode 13 must be fabricated with a low resistance value so as to prevent a voltage drop due to the resistance of these electrodes, and is usually made of Cr or Cu. These electrodes are covered with a dielectric layer (e.g., glass) 10, and a MgO (magnesium oxide) film 11 is formed as a protective film on the discharge surface. Third electrodes (addressing electrodes A1 to AM (where M is an arbitrary positive integer of 2 or more than 2) are formed on the back glass substrate 9 defined as the second glass substrate opposing the first glass substrate in such a manner as to orthogonally cross the sustain electrodes. The addressing electrodes A1 to AM are covered with the dielectric layer 10 to form barriers 6 thereon, and phosphors 12 having red, green and blue light emission characteristics are formed between the barriers 6. The two glass substrates are assembled in such a manner that the portions of ridges of the barriers 6 are in close contact with the surface of the MgO film 11.
The selective address discharge for selecting cells 5 is executed by selecting the addressing electrodes and the Y electrodes. The sustain discharge is effected between the X electrode and the Y electrode. In the panel 1 having such a construction, the sustain discharge is effected in narrower gaps between the adjacent sustain electrodes (which gaps are referred to as xe2x80x9cdischarge slitsxe2x80x9d) but is not effected in the broader gaps between the adjacent sustain electrodes (which-are referred to as. xe2x80x9copposite slitsxe2x80x9d).
The sustain electrodes are arranged on the entire surface:in the sequence of the X electrode 2 of the first display line, the Y electrode 3-1 of the first display line, the X electrode 2 of the second display line, the Y electrode 3-2 of the second display line, the X electrode 2 of the third display line, the Y electrode 3xe2x80x943 of the third display line, and so forth.
In FIG. 3, a timing chart useful for explaining the method for driving the plasma display panel according to the prior art when the-plasma display driving apparatus described above or the like is used, is illustrated.
The timing chart of FIG. 3 shows typically the configuration of frames necessary for forming the display screen of the plasma display panel and voltage waveforms of various driving voltage pulses for each of the electrodes. Generally, each frame is divided into a plurality of subframes for effecting multi-gradation display by setting mutually different light emission periods (strictly speaking, sustain discharge periods). Each of these subframes includes an initialization period (reset period) of the wall charges, an addressing period (abbreviated to xe2x80x9caddr. periodxe2x80x9d in FIG. 3) for executing selective write discharge (that is, selective address discharge) of display data for the selected cell after the execution of the reset period, and a sustain discharge period (abbreviated to xe2x80x9csust. discharge periodxe2x80x9d in FIG. 3) for repeatedly executing light emission display of the selected cell by utilizing the sustain discharge for sustaining this addressing discharge.
The explanation will be given in further detail. In the priming discharge period which is executed at least once for each frame, an all-cell write pulse having a voltage Vw higher than the discharge start voltage (i.e., discharge threshold voltage) is applied to the X electrodes only at the time of activation of the cells, and a voltage Vaw for stably executing surface discharge on the X and Y electrodes (e.g., Vw/2) is applied to the addressing electrodes so that the stable whole surface write/self-erase discharge can be carried out.
When the all-cell write pulse falls, the wall voltage due to the wall charges generated between the. X and Y electrodes becomes larger than the discharge start voltage, and the all-cell self-erase discharge occurs. Practically, however, all the wall charges having a negative polarity are not completely neutralized and a limited quantity of the wall charges remain in the cells. Here, the term xe2x80x9cwall charges having a negative polarityxe2x80x9d means the wall charges under the state in which the negative wall charges remains in the X electrode and the positive wall charges remain in the Y electrode. The all cell write discharge and the all-cell self-erase discharge due to the application of the all-cell write pulses have the function of generating background light emission of the display screen of the plasma display panel, and such a function is generally known as a xe2x80x9cpriming effectxe2x80x9d. In this sense, the all-cell write pulse is referred to as xe2x80x9cpriming pulsexe2x80x9d, and the all-cell write discharge by this priming pulse is referred to as xe2x80x9cpriming dischargexe2x80x9d.
In the second addressing period of each subframe, an address pulse having a voltage Va necessary for executing a write operation of display data by turning on the selected cell. (light emission display) is applied to the addressing electrode and an addressing pulse having a voltage Vx is applied to the X electrode. Further, the addressing pulse having a voltage Va is applied to the addressing electrode and then a scanning pulse having a voltage xe2x88x92Vy is applied to the Y electrode.
In the third sustain discharge period of each subframe, a train of sustain pulses having a voltage Vs for effecting sustain discharge to sustain the address discharge are applied to the X electrodes, and a train of sustain pulses, the phase of which is deviated by 180xc2x0 (xc2xd cycle) from that of the former sustain pulses and which have a voltage Vs, are applied to the Y electrode. Further, a voltage pulse having a voltage Ve is applied to the address electrode in synchronism with the rise of the first sustain pulse, and this voltage pulse is held until the sustain discharge period is terminated.
As described above, in the method for driving the plasma display panel according to the prior art shown in FIG. 3, the priming pulse larger than the discharge start voltage is applied once for each subframe (or for each frame when multi-gradation display is not effected) to the X electrode or to the Y electrode in the first reset period. Further, in order to insure stable address discharge in the addressing period, a predetermined voltage is applied to the addressing electrode in the first reset period of each subframe.
However, when the method for driving the plasma display panel described above is employed, the priming discharge must be generated at the time of turn-on of the power from the state in which any priming does not exist. Because the voltage necessary for this purpose is applied, a discharge larger than necessary develops in the priming discharge of the next subframe, and the rise of background light emission of the display screen takes place disadvantageously on the other hand, it would be possible, in principle, to execute the erase discharge by a large width erase pulse (i.e., long erase pulse) or a small width erase pulse (i.e., short erase pulse) for only those cells which have executed the sustain discharge. When the large width erase pulse or the small width erase pulse is used, however, a driving margin that represents the relationship of the voltage Vx of the addressing pulse and the voltage Vy of the scan pulse becomes extremely small, and the operation becomes unstable against the change with the lapse of time or the change of temperature.
The present invention has been completed in view of the problems described above, and is directed to-providing a method for driving a plasma display panel which can restrain the rise of background light emission brought forth by the occurrence of a discharge larger than necessary due to the priming discharge.
To accomplish the object described above, the present invention provides a method for driving an AC type plasma display panel which comprises arranging first electrodes and second electrodes in parallel with one another for each display line; arranging third electrodes in such a manner as to cross the first and second electrodes; and repeatedly executing light emission display by utilizing a selective address discharge for generating wall charges in cells selected by either one of the first and second electrodes and by the third electrode and a sustain discharge executed repeatedly for the cells in which the wall charges are generated.
In this method for driving the plasma display panel, each of a plurality of frames forming the display screen of the plasma display panel comprises a plurality of subframes each having predetermined luminance, and each of these subframes has a period in which the selective address discharge is executed and a period in which the sustain discharge is executed after the selective address discharge, and has, on the other hand, a period in which a priming discharge is executed at least once for each frame. Further, a pulse having a voltage higher than the priming pulse for executing a subsequent priming discharge is applied between the first and second electrodes so as to execute the priming discharge.
As described already, the priming discharge must be generated at the time of turn-on of the power source, that is, at the time of activation of the cells, from the state in which no priming exists. If a priming pulse having a low voltage is applied at this time, the priming discharge does not develop in some cases. Therefore, the method for driving a plasma display panel according to the present invention applies the priming pulse having a higher voltage than the voltages of subsequent priming pulses only at the time of activation of the cells, and in the subsequent priming discharge, a priming pulse having a lower potential is applied. Because the occurrence of a discharge which is larger than necessary is restrained in this way, background light emission can be reduced much more than in the prior art systems.